Low dn/dT optical adhesives

Abstract

Embodiments of an optical adhesive are provided. The optical adhesive includes about 20% to about 60% by volume of first monomers. The first monomers have at least two acrylate or methacrylate groups. The optical adhesive also includes about 40% to about 80% by volume of second monomers. The second monomers have at least one fluorine atom and at least one acrylate or methacrylate group. The optical adhesive has a refractive index of from about 1.40 to about 1.55, and in the temperature range of about 10° C. to about 85° C., the refractive index of the optical adhesive has a thermal drift dn/dT of less than about −4×10 −4 /° C. Embodiments of a mechanical joint between two optical fiber segments using the optical adhesive and embodiments of a method for joining two optical fiber segments are also provided.

What is claimed is: 
     
       1. An optical adhesive, comprising:
 20% to 60% by volume of first monomers, each of the first monomers comprising at least two acrylate or methacrylate groups; and 
 40% to 80% by volume of second monomers, each of the second monomers comprising at least one fluorine atom and at least one acrylate or methacrylate group; 
 wherein, upon curing, the cured optical adhesive has a refractive index of from about 1.40 to about 1.55; and 
 wherein, in the temperature range of about 10° C. to about 85° C., the refractive index of the cured optical adhesive has a thermal drift dn/dT of less than the absolute magnitude of |4×10 −4 /° C.| and the sign of that value is negative. 
 
     
     
       2. The optical adhesive of  claim 1 , further comprising about 1% to about 30% by weight of nanoparticles, the nanoparticles having, on average, a longest cross-sectional dimension of from about 5 nm to about 50 nm. 
     
     
       3. The optical adhesive of  claim 2 , wherein the nanoparticles are coated with at least one functional group. 
     
     
       4. The optical adhesive of  claim 2 , wherein the nanoparticles comprise silica. 
     
     
       5. The optical adhesive of  claim 1 , wherein:
 the first monomers comprise at least one of 1,4-butanediol diacrylate, 1,4-butanediol dimethacrylate, 1,3-butanediol diacrylate, 2,2,3,3-tetrafluoro-1,4-butanediol diacrylate, or 2,2,3,3-tetrafluoro-1,4-butanediol dimethacrylate; and 
 the second monomers comprise at least one of 1,1,1,3,3,3-hexafluoroisopropyl methacrylate, 2,2,2-trifluoroethyl methacrylate, 2,2,3,3,4,4,5,5-octafluoropentyl methacrylate, or 2,2,3,3-tetrafluoropropyl methacrylate. 
 
     
     
       6. The optical adhesive of  claim 1 , further comprising at least one of an adhesion promoter or an anti-oxygen inhibition additive. 
     
     
       7. The optical adhesive of  claim 1 , wherein at least a portion of the first monomers comprise at least one fluorine atom. 
     
     
       8. A mechanical joint, comprising:
 a first optical fiber segment having a first longitudinal axis, wherein the first optical fiber segment comprises a first core region and a first cladding region, and wherein the first optical fiber segment has a first end face transverse to the first longitudinal axis; 
 a second optical fiber segment having a second longitudinal axis, wherein the second optical fiber segment comprises a second core region and a second cladding region, wherein the second optical fiber segment has a second end face transverse to the second longitudinal axis, and wherein the first optical fiber segment is arranged with the second optical fiber segment such that the first longitudinal axis is coaxial with the second longitudinal axis and such that the first end face faces the second end face; 
 an optical adhesive bonded to the first end face and to the second end face, wherein the optical adhesive comprises a cured mixture of:
 20% to 60% by volume of first monomers, each of the first monomers comprising at least two acrylate or methacrylate groups; and 
 40% to 80% by volume of second monomers, each of the second monomers comprising at least one fluorine atom and at least one acrylate or methacrylate group; and 
 
 wherein a return loss across the mechanical joint is less than −40 dB as measured at a wavelength of 1310 nm. 
 
     
     
       9. The mechanical joint of  claim 8 , wherein the optical adhesive is also bonded to a least a portion of an outer surface of the first cladding region and to at least a portion of an outer surface the second cladding region. 
     
     
       10. The mechanical joint of  claim 8 , wherein the first cladding region defines a first outer diameter of the first optical fiber segment, the second cladding region defines a second outer diameter of the second optical fiber segment, and wherein the optical adhesive has a thickness of from about 1.01 to about 1.50 times the larger of the first outer diameter and the second outer diameter. 
     
     
       11. The mechanical joint of  claim 8 , wherein the optical adhesive fills a space between the first end face and the second end face, and wherein the space is less than 900 nm. 
     
     
       12. The mechanical joint of  claim 8 , wherein the first optical fiber segment and the second optical fiber segment each have a first refractive index, the optical adhesive has a second refractive index, and wherein the second refractive index is within about +/−5% of the first refractive index as measured at a wavelength of 1310 nm and at 25° C. 
     
     
       13. The mechanical joint of  claim 8 , wherein the optical adhesive further comprises about 1% to about 30% by weight of nanoparticles, the nanoparticles having, on average, a longest cross-sectional dimension of from about 5 nm to about 50 nm. 
     
     
       14. The mechanical joint of  claim 8 , wherein the optical adhesive further comprises at least one of an adhesion promoter or an anti-oxygen inhibition additive. 
     
     
       15. A method of joining two optical fibers, the method comprising the steps of:
 arranging a first optical fiber coaxially to a second optical fiber such that a first end face of the first optical fiber faces a second end face of a second optical fiber; 
 providing a liquid optical adhesive between the first optical fiber and the second optical fiber, wherein the optical adhesive comprises about 20% to about 60% by volume of first monomers and about 40% to about 80% by volume of second monomers, each of the first monomers comprising at least two acrylate or methacrylate groups, and each of the second monomers comprising at least one fluorine atom and at least one acrylate or methacrylate group; 
 exposing the liquid optical adhesive to ultraviolet light, thereby causing the liquid optical adhesive to cure into a solid optical adhesive. 
 
     
     
       16. The method of  claim 15 , wherein the first optical fiber and the second optical fiber each have a first refractive index, wherein the first monomers have a second refractive index that is higher than the first refractive index, wherein the second monomers have a third refractive index that is lower than the first refractive index, and wherein the method further comprises the steps:
 combining the first monomers with the second monomers to form the liquid optical adhesive with a fourth refractive index that is less than the first refractive index; and 
 wherein, after the exposing step, the solid optical adhesive has a fifth refractive index that is within 5% of the first refractive index. 
 
     
     
       17. The method of  claim 16 , wherein, during the exposing step, the fourth refractive index rises from about 0.01 to about 0.05 to the fifth refractive index. 
     
     
       18. The method of  claim 15 , wherein the liquid optical adhesive further comprises from about 1% to about 30% by weight of nanoparticles, the nanoparticles having, on average, a longest cross-sectional dimension of from about 5 nm to about 50 nm. 
     
     
       19. The method of  claim 15 , wherein:
 the first monomers comprise at least one of 1,4-butanediol diacrylate, 1,4-butanediol dimethacrylate, 1,3-butanediol diacrylate, 2,2,3,3-tetrafluoro-1,4-butanediol diacrylate, or 2,2,3,3-tetrafluoro-1,4-butanediol dimethacrylate; and 
 the second monomers comprise at least one of 1,1,1,3,3,3-hexafluoroisopropyl methacrylate, 2,2,2-trifluoroethyl methacrylate, 2,2,3,3,4,4,5,5-octafluoropentyl methacrylate, or 2,2,3,3-tetrafluoropropyl methacrylate. 
 
     
     
       20. The method of  claim 16 , wherein, in the temperature range of about 10° C. to about 85° C., a refractive index of the solid optical adhesive has a thermal drift dn/dT of less than the absolute magnitude of |4×10 −4 /° C.| and the sign of that value is negative.